The terms positive temperature coefficient (PTC) materials and, positive temperature coefficient of resistivity (PTCR) materials, as used herein, refer to materials that increase in electrical resistance when their temperature is raised. Polymers can be made electrically conductive by dispersing suitable amounts of conductive particles such as carbon black or fine metal particles. Polymeric compositions exhibiting positive temperature coefficient (PTC) behavior and devices incorporating the same have been used in many applications, especially in electronic industries, including uses as constant temperature heaters, over current regulators, and low-power circuit protectors. A typical use in a circuit is limitation of current, which is controlled by the temperature of a PTC element forming part of the circuit.
There are two major ways to produce such PTC compositions, the one is melt-extruding technology and the other is ink/coating technology. The melt-extruding technology is most popularly applied but the resulting articles may be inflexible and are generally unsuitable for configuration into the intricate or very thin shapes often desirable for use on flexible substrates or printed circuit boards. Over recent years, there has been particular interest in the ink/coating technology to produce polymeric PTC compositions. Reference may be made, for example, to U.S. Pat. Nos. 4,628,187, 5,181,006, 5,344,591, and 5,714,096, and Japanese Patent Publication Nos. 2008293672, 2009151976, and 2009199794. For these inks/coatings, the polymer resins, which are dissolved in suitable solvents, are the binders and the conductive particles are dispersed in the binders to obtain the inks/coatings. Various polymeric PTC compositions have been developed, however, most PTC compositions exhibit Negative Temperature Coefficient (NTC) characteristics of resistivity immediately after the PTC characteristics. This change from PTC behaviour to a strong NTC behaviour is often undesirable, and may cause self-burning in some cases. FIG. 1 shows a typical curve of the PTC composition described above. It indicates that when the temperature T, plotted on a horizontal axis, is in excess of 70° C., the PTC ratio RT/R25 is reduced, PTC ratio being a ratio between a resistance RT at a certain temperature T and a resistance R25 at a temperature of 25° C. The lower resistance leads to excessive current flow and the heating element is overpowered. Therefore, the NTC temperature region is a potential safety risk temperature region.
Efforts have been undertaken to reduce or eliminate the NTC effect. As disclosed in U.S. Pat. No. 5,227,946 and European Patent EP 0311142, reduction of the NTC effect in polymeric PTC compositions has been achieved by cross-linking the material. Most effective is post-cross linking after the forming step either by gamma radiation or accelerated electrons. Cross-linking in the melt also erases the NTC effect but negatively affects the PTC amplitude. In addition, the step of cross-linking the material increases the time and production costs for manufacturing the PTC composition.
U.S. Pat. No. 8,496,854 discloses a method to reduce the NTC effect without cross-linking the material. Their PTC compositions include a thermoplastic base resin, an electrically conductive filler and particles of a polymeric additive dispersed in the PTC composition; wherein the polymeric additive has a melting or softening temperature greater than the melting temperature of the thermoplastic resin, which helps reduce the NTC effect. The above PTC composition was produced by melt-extruding technology not like the ink/coating technology used in the present invention. In addition, the NTC effect is only reduced and not completely eliminated by the above method.
PTC materials are commonly used in a class of flexible heaters comprising an electrical resistor that is encapsulated between two polymer films. The resistor is typically an etched metal foil or, alternatively, an electrically conductive material, or ink, with fixed resistance and suitable properties for printing onto the polymer film. The foil is usually composed of Ni—Cr alloy; the ink is usually composed of an electrically insulating polymer with a dispersed, electrically conductive powder additive. The conductive powder can be composed of metals such as Ag, Au or Sn, electro-conductive ceramics such as WC, or carbon in various forms such as graphite or carbon black. Carbon is the most common. The powder additive, dispersed three-dimensionally throughout the polymer, has sufficient volume fraction in the polymer to reach its percolation threshold so as to form a contiguous, conductive network for electrical current to flow throughout the polymer. Both flexible heater types often have an adhesive on at least one outer side of the encapsulating film for affixing the heater to the part to be heated.
Polymeric PTC heaters form a subclass of the second type of flexible heaters described above. These flexible heaters are typically encapsulated in a polymer film like other heaters of the type and generate heat with an applied voltage, but have printed conductive polymer elements that can control current, hence control heating power. The temperature at which the electrical resistance starts to increase sharply is referred to as the switch temperature. A “switch temperature” or “switching temperature” as used herein refers to the temperature at which the PTC heater generates power only sufficient to maintain thermal equilibrium with its environment, i.e. the point at which the heater temperature neither rises nor lowers under its own power. Typical resistance magnification factors range from 5-15 at the switch temperature. Therefore, they are self-regulating. The switching mechanism is due to the polymer undergoing a phase change at around the switch temperature from its normal crystalline molecular structure to an amorphous molecular structure. Because the amorphous material phase has greater volume than the crystalline phase, its volume and thermal expansion interrupts the web of conductors dispersed throughout the polymer. Upon cooling below the switch temperature, the reverse phase-transformation takes place and the conductive network is re-established.
Polymeric PTC flexible heaters are used for a wide variety of applications where low temperature, controlled thermal gradients, uniform heating and localized temperature control are crucial, without requiring complex electronic controllers or feedback loops. PTC-based heater solutions can be found in many aspects of daily life from automotive applications (such as external mirror heaters, seat heaters, etc.), to structural home systems (including floor heaters, bed heaters, etc.) to small appliances (like rice or vegetable cookers).
Polymeric PTC flexible heaters are most commonly composed of a polymer with carbonaceous conductor in the form of carbon black or graphite. This is because polymeric PTC heaters with metal or conductive ceramics are more expensive and more difficult to deposit as films. However, carbon based polymeric PTC heaters suffer from several important operational technical problems.
One such problem is a low resistance magnification at the switch temperature, typically ranging from 5-15. This results in some power dissipation even at maximum resistance.
Another problem is the transition temperature region between the low resistance state and the high resistance state where only partial current flows. The transition region varies in width proportional to ambient temperature and the overall conditions for heat transfer. Therefore, the operational characteristics of the heater are determined by a multitude of design factors involving its physical environment. This affects the heater's power dissipation, the time-to-switch and the heater's hold current.
Another problem is that many PTC heaters exhibit resistance hysteresis when switched. This is observed as an increase in resistance from the starting resistance even hours after the switch event. Over time, the original resistance is approached but it may be days, months or years. Fortunately, the resistance is not cumulative over subsequent switch events so strategies can be taken to account for it.
Yet another problem is the heater's resistance recovery time after a reset event. The time is usually one to two minutes but can be longer depending on the heat transfer environment of the heater and its material design.
Still another problem and one that jeopardizes safety of the heaters is a strong NTC effect that most PTC materials exhibit at temperatures slightly above the temperature at which the strong PTC effect takes hold. The NTC effect causes a dramatic increase in current flow resulting in possible catastrophic failure of the component. This effect is quite problematic for applications of flexible PTC heaters because it can destroy the heater and even cause a fire.
The present invention provides a method to completely eliminate the NTC effect of the PTC composition produced by ink/coating technology.